In its elementary form, a printed circuit board includes, as a component, a dielectric layer of an epoxy resin-impregnated woven glass fiber which is known as "prepreg". On the opposite sides of the prepreg are bonded conductive copper foil sheets. Subsequently the copper, through a number of photographic processes, is etched to produce conductive paths on the surface of the prepreg layer. When so assembled, the lamination is often called a core or a board.
In the manufacturing process, it is not uncommon to assemble a stack of such boards, either of the elementary type described above, or with compound layers. The assembly is called a press lay-up and the stack is called a book. The entire book is heated and subject to pressure. After cooling and curing, the then bonded individual boards are separated from each other and subjected to further processing. This general technique is described in my earlier U.S. Pat. No. 4,875,283.
Of tantamount importance in the manufacturing procedure is the maintenance of cleanliness or lack of contamination of the copper foil sheets. This is true, whether or not the printed circuit board is a simple sandwich of outer layers of copper foil and one layer of prepreg or whether it's a compound board of numerous layers.
One of the chief causes of contamination is the presence of resin dust, fiberglass fibers, hair, bugs and various types of foreign material resulting from the earlier manufacture and cutting of the prepreg and shipping and storing of the prepreg. In the laying up of the book of printed circuit boards, great care is taken in removing the resin dust by various wiping techniques. Nevertheless, it is inevitable that some dust remains on the copper foil surfaces. The resin dust melts in the lamination process when heat and pressure is applied which results in dots or deposits on the surface of the copper.
Another cause of concern is the existence of pits or dents in the surface of the copper foil. This, too, can result from a spot of resin dust being on the foil during the heating and laminating process as it causes a depression in the copper. It can also result from handling of the very thin foil. To date, there is no sure method of eliminating the presence of resin dust, pits or dents although all efforts are made to alleviate the problem.
The presence of a pit or dent or unwanted deposit of molten resolidified resin on the surface of the copper sheet generally results in a defect in the finished product due to shorted or open conductive paths. In a finished printed circuit board are a series of parallel conductors. If there is a dent in the foil in the area where two conductors are to be formed in the imaging and the finishing process, the dent will become filled in and can cause an electrical short. Conversely, such a dent can also result in an open circuit if one of the conductors is discontinuous.
In today's technology, conductors are formed in the order of magnitude of 0.005 inches wide and generally with the same width spaces between two conductors. The desire and trend in the industry today is to make the conductors and the spaces between them even narrower as, for example, 0.00025 inches wide. If the surface of the copper is not perfect, either open or short circuits can be created, resulting in boards, which for the most part, are rejected. Sometimes boards are subject to reworking, but in higher technology usage, reworking is unacceptable and the boards become useless scrap.
Another cause of defects results from handling the foil. When the various layers of foil and prepreg are laid one upon another, their alignment is maintained by a series of tooling pins that extend upwardly from a tooling plate. The tooling plate is a thick steel plate constituting the bottom of the stack. Each layer, be it copper foil or prepreg or partially completed laminated cores of conductive material, are pre-drilled or pre-punched with holes in a predetermined pattern, generally adhering to industry standards of size and location. Each layer is then stacked manually over the tooling pins with the pins extending upwardly through the pre-drilled holes.
One side of the foil in the finished product becomes the exposed conductive path. The other side is generally treated in an oxiding process to produce a surface which has a roughness, is generally gray in color, and which permits better adherence to the molten resin in the bonding process. One weight of copper foil in use today is "half ounce foil" which means that 1/2 oz. of copper is distributed over 1 square foot. This results in a foil which is approximately 0.0007 inches in thickness. Foils of 1/4 ounce and 1/8 ounce are also used. It is obvious that the handling of a foil this thin is a difficult problem. Layers of such foil must be placed manually over the tooling pins. This can result in wrinkles, and wrinkles, too, can result in imperfect conductive paths in the finished product.
One of the objectives of this invention is to provide means for better foil handling, not only to prevent folds or wrinkles, but to maintain cleanliness. Each time the operator assembles the layers required to complete one printed circuit board, he must place a separator on the top of the pile and then proceed to layup on top of it the components of another board. He must, in the process, wipe the surfaces of not only the separator but each of the conductive foils also.
Another source of defective boards is resin bleed that takes place around the tooling pins.
As stated above, each of the layers is placed upon tooling pins which, of necessity, must be somewhat smaller than the holes pre-cut into the copper foil and the prepreg layers. In the process of applying pressure and heat to the book, molten resin bleeds around the tooling pins and can fill up the tooling holes in the prepreg and foil layers. It also can bleed laterally between the various layers, particularly between the copper foil and the separator plates. After curing, this resin must be removed or it will create a resist material in the etching process. Furthermore, it can subsequently flake off onto the surface of the copper foil. Not only does the resin bleed have adverse effects on the surface of the copper, but it makes disassembly of the boards difficult when solidified resin has built up around the pins. Removal of the boards from the pins is thus made difficult.
In light of the foregoing, there are three principle objects of the present invention. One is to provide means to facilitate the handling of the extremely thin tissue-like copper foils.
Secondly, it is an object to assure that the copper foil is maintained as uncontaminated as possible before and during the manufacturing process.
Thirdly, it is another object of the present invention to prevent the resin bleed that takes place around the tooling pins from flowing between layers of the boards.